A central processing unit (CPU) may access a peripheral device, such as a printer or video card using memory mapped input-output (MMIO). MMIO employs the same address bus to address both physical memory and I/O devices (e.g., physical peripheral devices)—the memory and registers of I/O devices are mapped to (associated with) memory address values, respectively. When an address is accessed by the I/O device, the accessed address may refer to a portion of physical RAM or to memory of an I/O device in an address space of the I/O device. CPU instructions employed to access the physical memory may be used for accessing peripheral devices. Each I/O device monitors the address bus of the CPU and responds to any CPU access of an address assigned to the I/O device, connecting the data bus associated with the CPU to the hardware register of the I/O device. To accommodate I/O devices, blocks of addresses used by the host I/O device may be reserved for I/O and are not be available for CPU physical memory.
Virtualization permits multiplexing of an underlying host machine (associated with a physical CPU) between different virtual machines. The host machine or “host” allocates a certain amount of its resources to each of the virtual machines. Each virtual machine may then use the allocated resources to execute applications, including operating systems (referred to as guest operating systems (OS) of a “guest”). The software layer providing the virtualization is commonly referred to as a hypervisor and is also known as a virtual machine monitor (VMM), a kernel-based hypervisor, or a host operating system of the host.
A virtual machine may access a virtual device using guest addresses. The hypervisor may expose a virtual device to the guest to permit the guest to execute instructions on the virtual device. If a virtual device is a virtual peripheral device, such as a virtual printer or virtual video card, the virtual device may be accessed using memory mapped input-output (MMIO).
When a guest address is accessed by the guest, the accessed guest address may refer to a portion of guest RAM or to guest memory of a virtual I/O device. Guest CPU instructions used to access the guest memory may be used for accessing virtual I/O devices. To accommodate virtual I/O devices, blocks of guest addresses used by the virtual devices may be reserved for I/O and are not be available for guest physical memory.
During execution of an MMIO-based instruction of the guest, the guest may attempt to access a guest address mapped to a memory space of the virtual device. The associated CPU typically translates the guest address to a hypervisor address by “walking” through page table entries of a guest page table located in the guest. In the guest page table, entries for guest addresses mapped to a memory space of the virtual device are typically marked as invalid to prevent the guest from directly accessing such addresses and trigger an exit to the hypervisor. On exit to the hypervisor, the hypervisor is usually provided by the CPU with only the guest address that the guest attempted to access. In order for the hypervisor to identify the associated MMIO instruction(s) and associated parameter(s) (e.g., one or more operands), the hypervisor typically re-executes the page walk and performs relatively computationally expensive instruction retrieval and instruction decoding operations.